OSMOSIS

Osmosis is a form of passive transport where the solvent water molecules move from an area of high water concentration to an area of low water concentration across a semi-permeable cell membrane. The solutes can not pass through the membrane.

Direction of Water Flow• The direction of osmosis, or water flow,

depends on the water concentration [H2O] on both sides of the membrane and the inability of the solute molecules to pass the membrane.

• Which way will water flow?ICF – 90% water

ECF – 95% water

• The water will flow into the cell because ICF [H2O] < ECF [H2O]

Calculating Water Concentration• Sometimes the [H2O] must be calculated by

subtracting, % [H2O] = 100% - solute%.• Which way will water flow?

Cell parts - 10%

ECF – 85% water

• The water will flow out of the cell as ICF [H2O] = 90% and ECF [H2O] = 85%

• And now which way will water flow, assuming that salt can not pass the semi-permeable membrane?

Cell parts - 20%

60% salt solution

Cell parts - 20%

60% salt solution

• What happens to the shape of the cell?

• From inside ICF [H2O] = 80% > ECF [H2O] = 40%

• The cell will shrink.

TONICITY• Tonicity is a term to describe what happens to

cells immersed in an external solution.• Tonicity is influenced only by solutes that can not

pass through the semi-permeable cell membrane.• An isotonic solution has the same water and

solute concentrations as the cell. The same amount of water flows in as out.

• Animal cells are normal but plant cells are soft or flaccid in an isotonic solution.

Hypertonic Solution

• A hypertonic solution means that there are more solutes in the external solution and a lower [H2O].

• A cell will lose water if immersed in a hypertonic solution.

• An animal cell will crenate or shrink.• A plant undergoes plasmolysis as the cell

membrane shrinks away from the cell wall.

Hypotonic Solution

• A hypotonic solution has less solutes than the cell and a higher [H2O] than the cell.

• Water flows into the cell.• Animal cells expand and can burst open or

lyse. This is from the Latin lysis for “rupture”.• Plant cells are comfortable in a hypotonic

environment as the cell is full of water and turgid.

Sodium – Potassium Pump

• This form of active transport is used by cells to form a concentration gradient with more sodium on the outside of the cell.

• The is crucial for nerve impulses and animal cell function.

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

1. Start the NaK Pump

• Three Na+ ions from the ICF attach to a special transmembrane enzyme protein.

• Also an ATP energy molecule is hydrolysed providing energy.

2. Pushing out sodium• The energy released by the ATP causes the enzyme

protein to change shape and push out the 3 Na+ ions to the ECF against the concentration gradient.

3. Moving K+• Now two K+ ions enter the enzyme protein from

the ECF.• The protein again changes shape and discharges

the two K+ ions to the ICF.• As a result there are more positive charges

outside the cell.• This ionic charge difference pulls water out of the

cell and keeps the cell from swelling and breaking.• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/

animation__how_the_sodium_potassium_pump_works.html